1. Field of the Invention
The present invention relates to an ultrasonic probe and, more particularly, to an ultrasonic probe useful in a medical diagnosing apparatus.
2. Description of the Related Art
An ultrasonic probe has an ultrasonic transmitting/receiving element having a piezoelectric element. The ultrasonic probe is used for imaging the internal state of a target by radiating an ultrasonic wave toward the target and receiving an echo reflected by an interface having a different acoustic impedance of the target. An ultrasonic imaging apparatus incorporating such an ultrasonic probe is applied to, e.g., a medical diagnosing apparatus for inspecting the interior of a human body and an inspecting apparatus for inspecting the interior of a metal welding portion.
As an example of the medical diagnosing apparatus, in addition to the tomographic image (B mode image) display of the human body, there has been recently developed an apparatus employing the "Color Flow Mapping (CFM) method" capable of performing two-dimensional color display of the speed of the blood flow of, e.g., the heart, liver, and carotid artery, by utilizing a Doppler shift in ultrasonic wave caused by the blood flow. The diagnosing performance has been remarkably improved by this medical diagnosing apparatus. The medical diagnosing apparatus employing the CFM method is used for diagnosis of all the internal organs, e.g., the uterus, liver, and spleen, of the human body. Further studies are in progress aiming at an apparatus capable of diagnosing coronary thrombus.
In the case of the former B mode image, a high-resolution image must be obtained at a high sensitivity so that even a small change to a morbid state and a body cavity at a deep location caused by a change in body can be clearly seen. In the latter Doppler mode capable of obtaining a CFM image, since the echo reflected by a small blood cell having a diameter of about several fm is used, the obtained signal level is lower than that obtained in the B mode image, and thus a higher sensitivity is required.
Conventionally, ultrasonic transmitting/receiving elements having the structures as follows are used in terms of their performance:
(1) Ultrasonic attenuation caused by irradiating a living body with an ultrasonic wave by an ultrasonic probe is about 0.5 to 1 dB/MHz.cm except in bones. Thus, in order to obtain a high-sensitivity signal from the living body, it is preferable to decrease the frequency of the ultrasonic wave radiated by the ultrasonic transmitting/receiving element. When, however, the frequency is excessively decreased, the wavelength of the frequency is increased to sometimes degrade the resolution. Therefore, an ultrasonic wave having a frequency of 2 to 10 MHz is usually radiated.
(2) The piezoelectric member of the ultrasonic transmitting/receiving element must be constituted by a material having a large electromechanical coupling coefficient and a large dielectric constant so that loss caused by cables and the stray capacitance of the apparatus is small and that the piezoelectric member be easily matched with a transmitting/receiving circuit. For this reason, the piezoelectric member is mainly constituted by a titanate lead zirconate (PZT)-based ceramic.
(3) An array-type ultrasonic probe constituted by arranging several tens to about 200 ultrasonic transmitting/receiving elements each having a strip-shaped piezoelectric member has a high resolution.
However, the conventional utrasonic probe has the following problems.
(a) The ultrasonic transmitting/receiving element usually radiates an ultrasonic wave by utilizing resonance of the vibration of the piezoelectric member in the direction of thickness. To decrease the influence of the attenuation in ultrasonic wave from a living body, the frequency of the ultrasonic wave must be decreased, as described above. To decrease the frequency of the wave, the piezoelectric member must be thicker. For example, in order to radiate an ultrasonic wave having a frequency of 2.5 MHz, the thickness of the piezoelectric member comprising the PZT-based ceramic must be set to 600 .mu.m in the direction of vibration. When the thickness of the piezoelectric member is increased in this manner, various problems occur. More specifically, to form a strip-shaped piezoelectric member from a PZT-based ceramic block, a dicer used in dicing a semiconductor silicon wafer and the like is used. When the thickness of the piezoelectric member in the direction of vibration is increased, the depth of cut when dicing is performed at a predetermined pitch is increased. If, for this reason, dicing is performed by using a thin blade, the cutting groove becomes oblique, the cut portion winds, or the piezoelectric member can be damaged. If dicing is performed by using a thick plate in order to avoid them, the cutting amount is increased. Then, since the size of the PZT-based ceramic blocks before dicing is predetermined, the area of the ultrasonic transmitting/receiving surface of each piezoelectric member is decreased. As a result, the sensitivity is decreased, and the side lobe (grating lobe) level is increased.
(b) When the array-type ultrasonic probe is brought into contact with the living body, since the diameter of the ultrasonic wave radiating surface cannot be increased, as the number of ultrasonic transmitting/receiving elements is increased, the impedance per piezoelectric member is increased, and matching with the transmitting/receiving circuit becomes difficult to obtain. Regarding matching, poor matching can be avoided by using the PZT-based ceramic having a large relative dielectric constant as the piezoelectric member. However, since the electromechanical coupling coefficient of the PZT-based ceramic is decreased when the relative dielectric constant exceeds 3,000, the sensitivity is decreased, thus causing another problem.
Regarding the problem (b) described above, matching with the transmitting/receiving circuit is obtained by forming the piezoelectric member as a multilayered structure or by incorporating an impedance converter. However, in a multilayered structure, although the transmitting sensitivity is increased in accordance with the number of layers, the receiving sensitivity is inversely proportional to the number of layers. Therefore, the application of the multilayered piezoelectric member is limited to special cases, e.g., a case wherein the piezoelectric member is smaller than usual and a case wherein the cable is long. When an impedance converter such as an emitter-follower is used, the size of the ultrasonic probe is increased, and the frequency band is narrowed due to the frequency characteristics inherent to the impedance converter.
It is known that a piezoelectric member constituted by a polymeric material, e.g., lead metaniobate, polyvinylidene fluoride, or a copolymer thereof, has a small frequency constant and that its thickness ca be smaller than that constituted by a PZT-based ceramic even if it has a low frequency. However, the polymeric material has a small dielectric constant and a small electromechanical coupling coefficient and is not thus practical.
As described above, when a high-sensitivity low-frequency driving ultrasonic probe which causes small attenuation in ultrasonic wave in a living body is to be obtained, if a conventional PZT-based ceramic is used, the thickness of the probe becomes large. For this reason, if a thin blade is used to perform dicing to obtain strip-shaped piezoelectric members, the cutting groove becomes oblique, the cut portion winds, or the piezoelectric member can be damaged. If dicing is performed by using a thick plate, as the cutting portion is increased, the area of the ultrasonic wave transmitting/receiving surface of each piezoelectric member is decreased. Then, the sensitivity is decreased, and the side lobe level is increased. Furthermore, when the thickness of the piezoelectric member is increased, the electric impedance is increased, and matching with the transmitting/receiving circuit becomes difficult to obtain.